Solution mining techniques for recovering soda ash (sodium carbonate) from subterranean trona ore deposits, such as described in U.S. Pat. No. 3,184,287 (issued to Gancy), are ordinarily operated in a cyclic manner. The mining solvent is typically regenerated from the mother liquor which remains after recovery of the solid sodium carbonate (monohydrate or anhydrous forms) from the withdrawn mining solution. High sodium carbonate recoveries are thus desirable so as to avoid carrying a large inventory of soluble sodium carbonate in the regenerated recycled mining solvent.
Solution mining of trona ore deposits which are contaminated with sodium chloride typically results in an appreciable concentration of sodium chloride in the withdrawn mining solution. This complicates the recovery of sodium carbonate in high yields because of the likelihood of co-precipitation of sodium chloride with the sodium carbonate product.
Prior art methods for crystallizing sodium carbonate from aqueous sodium carbonate solutions having an appreciable sodium chloride content do not teach means for recovering high yields of sodium carbonate, without co-precipitation of sodium chloride. U.S. Pat. Nos. 2,133,455 (issued to Keene et al) and 3,656,892 (issued to Bourne et al) disclose that anhydrous sodium carbonate may be crystallized from sodium carbonate solutions at temperatures around 105.degree.-110.degree. C. via the introduction of an additive like sodium chloride into the crystallizer. The introduction of sodium chloride into the crystallizer liquor lowers the monohydrate-anhydrous transition temperature and thereby yields anhydrous sodium carbonate, not the monohydrate, as the crystallized solid.
The method of the present invention provides for enhanced sodium carbonate recoveries from aqueous sodium carbonate solutions which also contain sodium chloride, without co-precipitation of salt which would contaminate the sodium carbonate product.